Method of cladding by hydrostatic pressure applied to heated units inside a cold liquid cladding apparatus



Feb. 22, 1966 M. GAUTHRON 3,235,958 METHOD OF CLADDING BY HYDROSTATIC PRESSURE APPLIED TO HEATED UNITS INSIDE A COLD LIQUID CLADDING APPARATUS Filed Sept. 24, 1962.

United States Patent 3 235,958 METHOD OF CLADDING BY HYDROSTATIC PRESSURE APPLIED TO HEATED UNITS INSIDE A COLD LIQUID CLADDING APPA- RATUS Maurice Gautln-on, Paris, France, assignor t0 Commissariat a lEnergie Atomique, Paris, France Filed Sept. 24, 1962, Ser. No. 225,599 Claims priority, application France, Oct. 9, 1961, 875,393 Claims. (Cl. 29474.3)

The present invention relates to the cladding of objects of various shapes, and especially of fuel elements intended for use in a nuclear reactor.

The methods hitherto employed consisted in carrying out the cladding operation by applying hydrostatic pressure at high temperature and under high gas pressure and for the most part encountered the following difficulties:

The production of high gas pressures;

The danger of explosion of the cladding bodies when their volume reaches a few litres, thereby making it necessary to operate the devices by remote control and to mount them inside reinforced chambers of concrete with roofs designed to blow off under the effect of explosion blast,

The need to employ a neutral gas of high purity in order to prevent corrosive action; it is in fact very difficult to obtain an apparatus which is designed for high pressures, which is also capable of holding a vacuum and which does not introduce any trace of oil in the fluid system in spite of the provision of an oil separator on the delivery side of the compressor; it is thus found necessary to introduce the element to be clad inside a cooling apparatus, in giving a rinse with neutral gas (argon, for example), and only subsequently to commence heating with an apparatus having a high thermal inertia and then allowing it to cool after the cladding process before withdrawing the clad element therefrom;

The difliculty of manufacturing pressurized furnaces which provide a sufficiently high temperature (800 C.) and which also withstand variations of pressure,

The difficulty of ensuring uniformity of temperature in a pressurized furnace by reason of the fact that considerable convection currents are always formed which convey the furnace heat to cold areas (cooled yoke pieces, for example) in spite of all the obstacles which are devised to prevent them; and

The difficulty of measurement of temperatures owing to the lack of uniformity which has been mentioned.

The purpose of the present invention is to overcome the disadvantages referred-to above by providing a process for cladding by means of hydrostatic pressure applied to heated units inside a cold-liquid cladding apparatus of either the oil type or water type.

The invention mainly consists in placing the element to be clad comprising a fuel plate between two cladding plates inside a flexible envelope, in surrounding said element with a heat insulating powder by filling said envelope with said powder, said powder being especially formed of a refractory metallic oxide, in sealing said envelope and in compacting said powder, around said element by applying externally a compressive force against said envelope, in removing from said envelope the block formed of compacted powder containing the element and in inserting said block inside a flexible container after rough machining of said compacted powder, in heating the unit formed by said container and said block in a furnace of known type until uniformity of temperature is obtained, and finally in effecting the cladding proper by placing said heated unit inside a coldliquid cladding apparatus also of known type.

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The invention can further comp-rise the following features considered either separately or in combination:

The flexible envelope is constituted by a bag of plastic material.

The heat insulating powder which is contained in the flexible envelope is compacted by the application of hydrostatic pressure;

The powder contained in the flexible envelope is compacted by means of a press;

The container is constructed of thin stainless steel sheet;

The cladding material is formed of a metal and the powder is formed of an oxide of the same metal as that of said cladding material;

The cladding material is formed of an alloy and the powder is formed of an oxide of that metal which forms the chief constituent of said alloy;

The powder is magnesium oxide;

The powder is aluminum oxide;

The powder is zirconium.

The method in accordance with the invention has the advantage of permitting the use of conventional apparatus, while the cold oil cladding process gives rise to no problem in the present state of technical knowledge. Moreover, an oil compressor is more simple than a gas compressor, no furnace is required in the cladding apparatus which can accordingly be of known type; the requi-site uniformity of temperatures presents no problem, all danger of explosion is eliminated; and, finally, no problem of temperature measurement arises after the cooling time of the container has previously been calibrated.

There is no longer any danger of corrosive action of the compression fluid and the pressure can be brought to higher values than in the case of a gas (pressures limited to 1,000 kilograms per square centimetre) since a working pressure of 5,-000 kilograms per square centimetre can be reached with simple equipment.

Such long operations as heating and cooling do not take place in the cladding apparatus, thereby avoiding the immobilization of costly equipment.

Reference being made to FIGS. 1 to 3 of the accompanying drawings, the description which follows below relates to one example, which is not given in any sense by way of limitation, of practical application of the method in accordance with the invention for the cladding of plate-type nuclear fuel elements.

FIG. 1 is a cross-section of a plate-type element which is located inside its cladding container.

FIG. 1a is a cross-section of a plate-type element which is located inside its cladding container.

FIG. 2 is a cross-section of an oil-type cladding apparatus which encloses said container.

FIG. 3 is a cross-section of a system of assembly of said plate-type elements.

FIG. 1 illustrates the element formed by a fuel plate 1 of the type formed of uranium alloyed with zirconium and enclosed within two cladding plates 2a and 2b which can be of zircalloy and which are joined together by means of a weld 3. The said fuel plate is placed in a flexible, envelope 4 which is closed at both ends and which can be formed of suitable plastic material, for example, said envelope being filled with insulating powder 5, for example zirconium powder which is then compacted, the compaction pressure being for example 800 kilograms per square centimetre.

FIG. 1a illustrates the compacted powder 5 surrounding fuel plate 1 and cladding plates 2a and 2b placed in a flexible container 4a which is closed at both ends and may be of thin stainless steel sheet.

In accordance with the method of the invention, the unit comprising container 4a, compacted powder and fuel element is placed inside a furnace of known type until uniformity of temperature is obtained, at 850 C., for example.

By virtue of the heat insulation of the compacted powder 5, approximately one quarter of an hour is then available before the temperature of the element falls by fifty degrees or so. If the optimum cladding temperature is 800 C., approximately one half hour is available between 850 C. and 750 C., which is usually a sufficient period of time for the purpose of carrying out the cladding process and obtaining appropriate diffusion.

As can be seen in FIG. 2, the cladding apparatus 6 is filled with cold compression oil 7 in which are immersed the container 4 and its heated contents. The conduit 8 leads to an oil compressor of known type. The thickness 9 of the compacted powder can be of the order of a few centimetres.

When the diffusion is suitably initiated in the cladding apparatus 6, said diffusion is carried to completion by means of a subsequent thermal treatment at an appropriate temperature and in a controlled atmosphere.

In a general manner, the optimum temperature for the hydrostatic compression step as well as the length of time of said compression is pre-determined by means of systematic tests and by inspecting the structure of the parts formed by Various conventional methods such as micrography, ultrasonic vibration, etc.

Should the compression time prove to be insufficient, the process of compression inside the cladding apparatus 6 could be carried out a number of times after the element has been first reheated a corresponding number of times.

The plate-type elements produced as a result of the utilization of the devices in accordance with FIGS. 1 and 2 can then be joined by welding.

However, it is also possible to effect the assembly of a number of plate elements and the cladding of each of said elements in a single operation. To this end, the plate elements are arranged as shown in FIG. 3 by fitting keys between said elements and by placing plates 11 against the two end elements. The combined unit thus formed is introduced inside a box or casing and surrounded with powder which is then compacted; and finally, the said unit is placed in position inside the hydrostatic cladding apparatus which effects the cladding and assembly in a single operation.

t will naturally be understood that the present invention is not in any sense limited to the process which has been described in the foregoing example but is intended on the contrary to include all alternative forms within its scope.

It can additionally be applied to purposes other than those referred-to above, for example to compacting or stamping operations performed with the use of heat.

What I claim is:

1. Method of cladding by hydrostatic pressure consisting of the steps of placing an element to be clad consisting of a fuel plate surrounded by cladding material inside a flexible envelope, then filling said envelope with a heat insulating powder and surrounding said element with said powder, said powder being a refractory metal, then sealing said envelope, then compacting said powder around said element by applying externally a compressive force against said envelope, then removing from said envelope the block of compacted powder containing said element, then inserting said block in a flexible container after rough machining said compacted powder, then heating the unit formed by said container and said block until uniformity of temperature is obtained and then applying a cold liquid hydrostatic pressure to said heated unit to clad said element.

2. Method as described in claim 1, said flexible envelope being a bag of plastic material.

3. Method as described in claim 1, said powder in said flexible envelope being compacted by hyrostatic v pressure.

4. Method as described in claim 1, said powder in said flexible envelope being compacted by a press.

5. Method as described in claim 1, said container being of thin stainless steel sheet.

6. Method as described in claim 1, said cladding material being a metal and said powder being an oxide of the same metal.

7. Method as described in claim 1, said cladding mate- -rial being an alloy and said powder being an oxide of 1 aluminum oxide.

10. Method as described in claim 1, said powder being zirconium.

References Cited by the Examiner UNITED STATES PATENTS 1,081,618 12/1913 Madden.

2,932,882 4/1960 Kelly 2942l XR WHITMORE A. WILTZ, Primary Examiner.

J. C. HOLMAN. P. M. COHEN. Assistant Examiners. 

1. METHOD OF CLADDING BY HYDROSTATIC PRESSURE CONSISTING OF THE STEPS OF PLACING AN ELEMENT TO BE CLAD CONSISTING OF A FUEL PLATE SURROUNDED BY CLADDING MATERIAL INSIDE A FLEXIBLE ENVELOPE, THEN FILLING SAID ENVELOPE WITH A HEAT INSULATING POWDER AND SURROUNDING SAID ELEMENT WITH SAID POWDER, SAID POWDER BEING A REFRACTORY METAL, THEN SEALING SAID ENVELOPE, THEN COMPACTING SAID POWDER AROUND SAID ELEMENT BY APPLYING EXTERNALLY A COMPRESSIVE FORCE AGAINST SAID ENVELOPE, THEN REMOVING FROM SAID ENVELOPE THE BLOCK OF COMPACTED POWDER CONTAINING SAID ELEMENT, THEN INSERTING SAID BLOCK IN A FLEXIBLE CONTAINER AFTER ROUGH MACHINING SAID COMPACTED POWDER, THEN HEATING THE UNIT FORMED BY SAID CONTAINER AND SAID BLOCK UNTIL UNIFORMITY OF TEMPERATURE IS OBTAINED AND THEN APPLYING A COLD LIQUID HYDROSTATIC PRESSURE TO SAID HEATED UNIT TO CLAD SAID ELEMENT. 